Laser structures

ABSTRACT

Laser structures for generating and amplifying laser light, each such structure embodying an elongated core of solid laser material embedded within a cladding of solid light-transmitting material and within which cladding is also embedded a plurality of elongated flashtube chambers arranged in symmetrical relation to said core.

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5197A XR 3,646,472 3,646,472

Cooper et al. Feb. 29, 1972 [54] LASER STRUCTURES 3,356,966 12/1967Miller ..331/94.5

E Inventors: Alfred R. Cooper, Cleveland Heights 3,413,567 11/1968l-lannwacker et al. "331/955 1 Ohio; Charles Gilbert Young, Storrs,OTHER PUBLICATIONS Con 7 Ross, Dieter, Analysis of Room- Temperature cwRuby [73] A IEH I American p ic 'P Laser...The Ruby Laser as a ThermalLens. Journal of Ap- Southbridge, Mass. plied Physics, Vol. 37 No. 9,August 1966. pp. 3587- 3594.

[22] plied: 1967 Primary Examiner-Ronald L. Wibert [21] Appl. No.:672,307 Assistant Examiner-T. Major 4 Att0rneyJ. Albert Hultquist andNoble S. Williams [52] US. CL... ..33l/94.5 57 ST C [51] Int. Cl".....H0ls 3/02 5s 1 Field of Search ..331/94.5 Laser Structures forgefleratmg and amphfymg laser each such structure embodying an elongatedcore of solid laser 56 R f C-ted material embedded within a cladding ofsolid light-trans- 1 mm mitting material and within which cladding isalso embedded a UNITED STATES PATENTS plurality of elongated flashtubechambers arranged in symmetrical relation to said core. 3,533,01310/1970 Seltz 3,455,666 7/1969 Bazinet ..331/94.5 2 Claims, 5 DrawingFigures PATENTEBFEB 29 I972 INVENTORS ALFRED R. COOPER BY CHARLESG/LBERT YOUNG ATTORN Y More particularly, the invention relates tounitary laser i structures formed of solid materials and of suchimproved construction and arrangement as to provide high operatingefficiencies, improved operating characteristics and better heatdissipation than have been possible heretofore during continuous orrapid intermittent use of fundamentally similar structures of earlierdesign.

The improved laser constructions of the present invention, in fact,comprise, in each case, an elongated main body member or component whichis formed by a core of solid active moldable transparent laser materialhaving a predetermined refractive index value embedded within a claddingmaterial of substantially the same, or very nearly the same, refractiveindex value, said cladding also having embedded therein a plurality ofelongated hollow bores which are symmetrically arranged about the coreand extend throughout the length of the component in adjacentsubstantially parallel relation to said core and serve as flash tubechambers in such a way as to effect in said laser structure a closeroptical coupling, a more stable physical arrangement of parts as well asa more evenly balanced arrangement insofar as thermal gradients withinthe structure are concerned, a more efficient use of the pumping opticalenergy and care for the heat generated than has been possible previouslyin said earlier laser structures of unitary design.

Even though unitary laser structures employing, in each case, anelongated core of solid moldable laser material and a flashtube chamberwithin a common cladding of transparent moldable material and disposedin adjacent side-by-side relation to each other are known, and suchstructures have already provided relatively high operating efficiencieswith good heat dissipation during rapid intermittent laser operationthereof, nevertheless, they have not been satisfactory as might bedesired.

For example, not only did such unitary laser structures of earlierdesign have their operating characteristics change during use thereofbut also the amounts of such changes varied differently under differentoperating conditions and during different periods of use. Also,relatively high-current density was required for operating the singleflashtube thereof.

The improved constructions and arrangements of the present invention, onthe other hand, are such as to not only remove the undesirablelimitations of said earlier unitary laser structures but also are suchas to materially improve the operating characteristics and heatdissipation thereof.

In fact, it has now been found that by the use of a plurality ofproperly formed and properly finished hollow bores within the claddingmaterial of the improved laser component to serve as pumping lightsource chambers, instead of a single chamber as previously, and by soembedding these several bores within the cladding material in asymmetrical arrangement about the core and is closely adjacent butspaced relation to each other and to said core, and with each chamberarranged to extend in a generally parallel relation to said core, notonly can a stable or balanced condition for the improved laser structurebe obtained but also more efficient use of the available pumping opticalenergy can be had.

Not only will this balanced stable condition be maintained even thoughthe temperatures within the structure may change appreciably but alsomaterials of differing properties may be used without affecting thisbalanced condition. Also, all parts of the core of the laser structurewill be more unifonnly irradiated by the plurality of symmetricallyarranged flashtube chambers and additionally lower current densities forthe plural flash tube means may be used.

An additional advantage afforded by the use of the plurality ofsymmetrically arranged flash tubes is that a greater total surface areais provided for cooling purposes; and such is important since most ofthe heat generated within the laser structure during use thereof isgenerated within the individual flashtube cavities. v

Furthermore, when a symmetrical arrangement of several flashtubes isemployed at high repetition rates, the cores-of laser material may beoperated at higher operating temperatures (since it is more completelysurrounded by the hot flashtubes) and when such is the case more pumpingoptical energy will be absorbed by the laser material, due to thebroadening of the absorption bands of the laser material as thetemperature thereof increases.

In one modified form of the invention, by proper symmetrical arrangementof the-plurality of flashtube chambers about a single core of lasermaterial and by proper peripheral shaping of the solid claddingmaterial, in a more or less scalloped manner, not only will goodinternal reflection of the pumping optical energy be provided at theouter surface of the cladding material but also the arrangement will besuch that no single flashtube of the group will, to any appreciabledegree, see" directly another flashtube of the group. Accordingly,substantially no absorption of pumping energy from one flashtube byanother flashtube will occur.

It is, accordingly, a principal object of the present invention toprovide a unitary laser structure in the form of a relativelydiam-elongated main body member or component comprising a central coreformed of a solid transparent moldable active laser material and acladding of solid light-transmitting moldable material in surroundingcontacting relation to the sidewall portions thereof and to providewithin said cladding material in closely adjacent and substantiallyparallel relation to said elongated laser core a plurality of flashtubechambers disposed in a symmetrical arrangement about said laser core,whereby physical distortions of the structure due to uneven thermalexpansion, optical distortions within the structure clue to dissimilartemperature gradients in difierent internal parts of the core andcladding, and the like, will be avoided.

It is an object of the present invention to provide in a unitary laserstructure of the above character, and employing a plurality of flashtubechambers embedded within the cladding material thereof, a sufficientnumber of chambers and to so space said chambers relative to each otherand so shape the reflective outer surfaces of the cladding materialenclosing said chambers that said flastubes together will more uniformlyilluminate all parts of said laser core than has been possibleheretofore.

It is a further object of the present invention to provide for anelongated unitary laser structure of the above character, an outerperipheral shape or configuration for said cladding material which issuch that substantially no pumping optical energy from one flashtubechamber will be allowed to pass directly to an adjacent flashtubechamber.

It is also an object of the invention to provide in a unitary laserstructure of the above character a multiplicity of symmetricallyarranged tlashtube chambers embedded within the cladding material insuch a manner as to provide a balanced condition" substantially freefrom physical distortions due to thermal changes or the like andsubstantially free from optical distortions due to dissimilar internaltemperature gradients in different parts of the laser structure, and tohave the number of flashtube chambers employed sufficient to enable theflashtubes to operate at lower current density while being nearlynonabsorbent of the pumping optical energy emitted by adjacentflashtubes and while being better spectrally matched to the pumpingrequirements of the laser material, whereby improved operatingcharacteristics may be obtained. Other objects and advantages of theinvention will become apparent from the description which follows whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a side elevational view, partly broken away, of a laserstructure embodying the present invention;

FIG. 2 is an enlarged transverse section taken substantially uponsection line 2-2 of FIG. 1 and looking in the direction of the arrows;and

FIGS. 3, 4 and 5 are cross-sectional views showing modified forms of theinvention.

Referring to the drawing in detail and in particular FIGS. 1 and 2, itwill be seen that a unitary laser device embodying the invention isindicated generally at and that this device or structure comprises athin, elongated main body member or component 12 formed by a centralcore 14 of solid active laser material and about which is disposed acladding 16 of solid light-transmitting material. The material formingthe core has a predetermined refractive index and the cladding materialpreferably has the same, or very nearly the same, refractive index asthat of said core. Preferably, this solid laser material will be glass,such as a neodymium-doped barium crown laser glass, and the surroundingcladding material will be a suitable selectively absorbing glass such asa samarium-doped glass, adapted to function therewith, absorbing opticalenergy at the emission wavelength of the laser material on the one handwhile, on the other hand, having good light-transmitting propertiesinsofar as optical energy at the pumping wavelengths of the lasermaterial are concerned.

Electrodes for energizing the flashtubes are indicated at and 17 in FIG.1 and highly reflective coatings upon the ends of the laser core 14 fordefining a resonant laser cavity are shown at 19a and 1911. Of course,in usual fashion, one of these coatings, at least, would be slightlytransmissive, if desired. Or, if desired for easier alignment andadjustment into parallelism, such reflective end coatings forestablishing and defining the limits of the resonant laser cavity couldbe placed outwardly of the ends of the laser rod along central axis 12aand perpendicularly disposed relative thereto. Furthermore, if thestructure is to be operated as a laser amplifier, rather than a lasergenerator, such end reflectors would be omitted entirely. In such cases,it is possible that sloping end surfaces of controlled angularity so asnot to detrimentally internally reflect laser light might be used.

The cladding material 16 is, in fact, disposed in intimate opticalcontact with the smooth sidewall portions 18 of the elongated core 14 oflaser material throughout substantially the entire length thereof, sothat substantially all pumping optical energy travelling within saidcladding material 16 in such directions as to impinge upon the interface18 between the core and cladding will pass therethrough and into saidlaser material without experiencing any appreciable reflection at thisinterface.

Also contained within this cladding material 16, as shown in FIG. 2, area pair of similar flashtube chambers 20a and 20b which are positionedclosely adjacent the central core 14 and are arranged to extend indirections substantially parallel to the core; and it will be noted thatthese chambers are purposely arranged so as to be in diametricallyopposed relation to each other at opposite sides of the elongated core14. Also, the cladding material 16 is carefully controlled as to itsperipheral shape or contour and its exposed outer side wall portions 16aare smooth and coated, as indicated at 22, with a layer of highlyreflective material so as to reflect light rays impinging thereongenerally back toward core 14. Thus, a balanced condition as concernsthermal expansion and the like during operation of the structure will beprovided. Also, it should be noted that the arrangement is such thatmost of the optical energy generated within the flashtube chamber 29a,for example, will be directed so as to travel with a minimum ofreflection towards the core of laser material, as indicated by straightarrow A and reflected arrows B and C.

Since most of the heat generated within such unitary laser structuresduring operation thereof occurs within the flashtube chambers, thehighest temperatures within the cladding material 16 will occur in areasclosely adjacent these chambers, while in parts of the cladding materialfurther removed from said flash tubes, lower temperature will beproduced. Such is of particular interest in unitary laser devices orstructures of the present invention wherein it is intended to operatesaid devices intermittently at high repetition rates and the highest ofefficiencies are desired.

in unitary laser structures of earlier design wherein a single flashtubechamber was used, unbalanced conditions have occurred. This has beenreferred to, at times, as a bi-metal effect since two differentmaterials have been responsible in producing such a condition. Thermalexpansion has caused the laser body to bend or distort somewhat whilethe structure is operating. Thus, for example, in a Fabry-Perot type oflaser cavity, this bending has affected the operating conditions thereofdetrimentally. The reflecting surfaces upon the opposite end surfaces ofthe laser body have not retained their initial highly parallel conditionand undesired effects have resulted. Even when such reflectors fordefining the laser cavity were removed from the ends-of the elongatedlaser body and placed in an aligned facing relation outwardly thereof,the unitary laser structures of earlier design have exhibiteddetrimental effects, due to uneven thermal expansion of the dissimilarmaterials of core and cladding, or nonsymmetrical configuration thereof.

In the structural arrangement of FIGS. 1 and 2, it will be appreciatedthat the two flashtube chambers Zlia and 20b are similarly disposedadjacent and relative to the laser core 14, are in directly oppositepositions relative to the core 14 and the adjacent cladding wallthicknesses are the same; with the result that substantially equivalentthermal expansion condi tions, and the like, effecting the various partsof the laser structure will occur during laser operation and this willbe so whether the flash tubes of the different arrangements of thepresent invention are arranged to be operated simultaneously orsuccessively in rapid succession. Even though heat within the claddingor within the laser rod, or within both, may cause expansion of thesolid core and cladding materials thereof, nevertheless, the core 14 oflaser material will remain substantially straight and stable and,accordingly, very little, if any, effect due to heat building up in thestructure during high repetition rate operation thereof will occur.

In FIGS. 3 and 4, modified forms of the invention are shown. in thesetwo Figures, slightly different cross sections of laser structures areshown. In one Figure, a group of three flashtube chambers 3M2, 30b and300 are employed within the cladding 31 and are symmetrically arrangedabout the laser core 32, and separated by deep recesses 33, while, inthe other, a group of four chambers 34a, 34b, 34c, and 34d areemployedwithin the cladding 36 surrounding core 37 and are separated byrecesses 38. Nevertheless, in both arrangements, a plurality offlashtube chambers are employed about the laser body and same areequally spaced relative thereto as well as equally spaced from adjacentflashtube chambers of the group. Thus, in both modifications of FIGS. 3and 4, even though it cannot be said that they are each provided withdiametrically opposed flashtube chambers, nevertheless, both areprovided with a balanced" or symmetrical arrangement of parts, and thusthermal expansion during operation of the laser structure will notproduce bending or distortion of the laser structure, md will notproduce dissimilar temperature gradients in different parts of thestructure which might otherwise detrimentally affect laser operation ofthe structure.

An important consideration insofar as the flashtube groupingarrangements of FIGS. 3 and 4 are concerned is that each individualflashtube chamber is substantially optically isolated from the otherchambers of the structure. Thus, pumping optical energy emitted by anyflashtube chamber thereof will be directed substantially entirely towardthe laser core, and none of this energy will be allowed to travel towardother chambers of the group so as to be absorbed thereby. Thus, it canbe said that in such arrangements no flashtube chamber of the group isallowed to see" any other chamber of the group. On the other hand, agreat deal of the exterior exposed surface of the cladding materialsurrounding these chambers will be available for better heatdissipation.

Any reasonable number of flashtube chambers desired may be used in thepresent invention and disposed in equally spaced relation to one anotherabout an elongated central laser core and within the cladding materialtherefor. Such a mu: nian modified arrangement employing a larger numberof flashtube chambers is disclosed in FIG. 5. Here a laser structureemploying six separate similar flashtube chambers aide-40f is shownwithin cladding 42 surrounding central core 43. While in cross-sectionalarrangements shown in FIGS. 3 and 4 have been provided with relativelydeep recesses 33 and 3%, respectively, between adjacent flashtubechamber areas, and each recess is, in efiect, well rounded at its innerpart to avoid any sharp angles or the like which might provide planes ofweakness within the cladding structure, it will be appreciated,nevertheless, that these recesses tend to shield each flashtube fromadjacent flash tubes at opposite sides thereof. Thus, not only will mostof the light being radiated by each flashtube be caused to travel towardthe laser core but also almost no reabsorption of pumping optical energyby an adjacent flashtube will occur.

In the modified construction of HG. 5, on the other hand, wherein alarge number of flashtube chambers are employed, a scalloped effect isprovided by shallow pointed recesses 44 between adjacent flashtubesections. These shallow recesses do not help to shield one flashtubefrom an adjacent flashtube; nevertheless, the cladding material adjacentthese recessed areas does help reflect and direct pumping energy towardthe central core 42. On the other hand, since a relatively large numberof flashtube chambers are here being used, a more uniform illuminationof the entire laser core surface during operation thereof will occur, amore desirable absorption of pumping optical energy by the core due to abroadening of the absorption bands of the laser material at highertemperatures will occur, lower current densities can be used, a betterspectral matching between the flash tube output and the pumpingwavelengths needed for the laser material being used can be had at lowercurrent densities, and at lower current loading the flashtubes will bemore transparent to their own light and thus will not detrimentallyabsorb pumping light from other adjacent flash tubes of the group whichthey might see."

While in FIG. 5, a group of six flashtube chambers in a single unitarystructure has been shown, it should be appreciated greater numbers ofchambers in single structures are possible and practical. If desired, adozen or even more chambers in a single laser suucture could be employedwith advantageous results. In such an arrangement employing a number offlash tubes, such as one using twelve, it might be preferable to outerpolished side wall portions 16a of the cladding material but also abouteach of these highly reflective coatings will be disposed a relativelyheavy layer of metallic material of good heat conductivity, such ascopper, as shown at 24 in FIG. 2, and such may be conveniently placedthereon as by an electrochemical deposition process or the like. A

It should be appreciated, of course, that not only are the severaladvantages due to high pumping efficiencies and high heat conductivityand dissipation mentioned in the unitary laser construction disclosed insaid earlier-filed copending application realized by the instantinvention but additionally, due to the plurality of symmetricallyarranged flashtube chambers providing a balanced configuration andcontrolled contour shapes in the instant disclosure, no so-calledbimetal bending effects will be produced, higher operating temperaturesmay be employed, better spectral matching realized, greater heatdimipation accomplished and better operating characteristics realizedthan have been possible heretpfore.

Having described our invention, we claim: 1. A unitary laser structurecomprising a relatively thin,

elongated core formed of a solid transparent active laser material and acladding of solid light-transmitting material disposed in surroundingcontacting relation to the side wall portions of said core throughoutsubstantially the entire length thereof, said laser material having apredetermined refractive index value and said cladding material having arefractive index value which is of very nearly the same value as that ofsaid laser material, said cladding material having a plurality ofsimilar elongated cylindrical chambers formed therein in like closelyadjacent but spaced relation to said core in which a gas be placed toform flashtubes to supply optical pumping energy, and each chamber beingarranged to extend in a direction substantially parallel to the axis ofsaid core throughout the length of said cladding, said plurality ofchambers being equally peripherally spaced from one another about saidcentral core, so as to effect a symmetrical arrangement relative to saidcore, longitudinal recesses being formed in said cladding materialintermediate adjacent chambers and extending throughout the length ofsaid cladding material, said recesses being of sufficient depth to formobstructions to prevent the transmission of any substantial amount ofpumping optical I energy from one chamber to another whereby distortionsof operate certain groups of these 12 flashtubes alternately oraluminum, will be employed in contacting relation about the iiii

1. A unitary laser structure comprising a relatively thin, elongatedcore formed of a solid transparent active laser material and a claddingof solid light-transmitting material disposed in surrounding contactingrelation to the side wall portions of said core throughout substantiallythe entire length thereof, said laser material having a predeterminedrefractive index value and said cladding material having a refractiveindex value which is of very nearly the same value as that of said lasermaterial, said cladding material having a plurality of similar elongatedcylindrical chambers formed therein in like closely adjacent but spacedrelation to said core in which a gas be placed to form flashtubes tosupply optical pumping energy, and each chamber being arranged to extendin a direction substantially parallel to the axis of said corethroughout the length of said cladding, said plurality of chambers beingequally peripherally spaced from one another about said central core, soas to effect a symmetrical arrangement relative to said core,longitudinal recesses being formed in said cladding materialintermediate adjacent chambers and Extending throughout the length ofsaid cladding material, said recesses being of sufficient depth to formobstructions to prevent the transmission of any substantial amount ofpumping optical energy from one chamber to another whereby distortionsof said laser structure due to uneven thermal expansion or the likeduring laser operation will be avoided, and a highly reflective layer ofmaterial disposed upon and surrounding the outer surface of saidcladding material throughout the greater part of the length thereof. 2.A laser structure as defined in claim 1 and wherein said claddingmaterial is highly transparent to optical energy at the pumpingwavelengths of said laser material but is absorptive of optical energyat the emission wavelength of said laser material.